Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a permanent record of the charge image.
Various types of photoconductive insulating elements are known for use in electrophotographic imaging processes. In many conventional elements, the active components of the photoconductive insulating composition are contained in a single layer composition. This composition is typically affixed, for example, to a conductive support during the electrophotographic imaging process.
Among the many different kinds of photoconductive compositions which may be employed in typical single active layer photoconductive elements are inorganic photoconductive materials such as vacuum evaporated selenium, particulate zinc oxide dispersed in a polymeric binder, homogeneous organic photoconductive compositions composed of an organic photoconductor solubilized in a polymeric binder, and the like.
Other especially useful photoconductive insulating compositions which may be employed in a single active layer photoconductive element are the high-speed heterogeneous or aggregate photoconductive compositions described in Light, U.S. Pat. No. 3,615,414 issued Oct. 26, 1971 and Gramza et. al., U.S. Pat. No. 3,732,180 issued May 8, 1973. These aggregate-containing photoconductive compositions have a continuous electrically insulating polymer phase containing a finely-divided, particulate, co-crystalline complex of (i) at least one pyrylium-type dye salt and (ii) at least one polymer having an alkylidene diarylene group in a recurring unit.
In addition to the various single active layer photoconductive insulating elements such as those described above, various "multi-active-layer" photoconductive insulating elements, i.e., those having more than one active layer have been described in the art. One useful type of "multi-active-layer" photoconductive element is described in Hoesterey, U.S. Pat. No. 3,165,405 issued Jan. 12, 1965, at column 2, lines 6-20 thereof. As described in this patent, photoconductivity is achieved by applying a uniform positive charge to the surface of an element containing two layers of zinc oxide, a sensitized zinc oxide bottom layer and an unsensitized zinc oxide upper layer, and then exposing the sensitized bottom layer to a pattern of activating radiation. Photoconductivity is produced in the element by the electrical interaction of the two zinc oxide layers. The sensitized zinc oxide bottom layer generates photoelectrons, i.e. negative charge carriers, and injects these charge carriers into the unsensitized zinc oxide upper layer which accepts and transports these charge carriers to the positively charged surface of the photoconductive element.
The concept of using two or more active layers in a photoconductive insulating element, at least one of the layers designed primarily for the photogeneration of charge carriers and at least one other layer designed primarily for the transportation of these generated charge carriers, has been discussed in the patent literature. Such multi-active-layer photoconductive elements are sometimes referred to hereinafter simply as "multi-active" photoconductive elements. In addition to the above-noted Hoesterey patent, a partial listing of representative patents discussing or at least referring to "multi-active" photoconductive elements includes: Bardeen, U.S. Pat. No. 3,041,166 issued June 26, 1962; Makino, U.S. Pat. No. 3,394,001 issued July 23, 1968; Makino et. al. U.S. Pat. No. 3,679,405 issued July 25, 1972; Hayaski et. al., U.S. Pat. No. 3,725,058 issued Apr. 3, 1973; Canadian Pat. No. 930,591 issued July 24, 1973; and Canadian Pat. Nos. 932,197-199 issued Aug. 21, 1973; and British Pat. Nos. 1,337,228 and 1,343,671.
Although there has been a fairly extensive description of specific types of multi-active photoconductive insulating elements in the literature, various shortcomings still exist in these elements so that there is a need to investigate alternative kinds of multi-active elements. For example, the multi-active elements described in the aforementioned Hoesterey patent suffer from the disadvantages of using generally low speed and difficult to clean zinc oxide materials in both active layers of the element. Other multi-active elements such as those described in Canadian Pat. Nos. 930,591 and 932,199 appear to be primarily designed for use in a positive charging mode of operation and therefore may not generally be suitable for use in an electrophotographic process in which a negative charging mode is employed. Furthermore, the type of multi-active elements described in U.S. Pat. Nos. 3,041,166; 3,394,001; 3,725,058 and Canadian Pat. No. 930,591, which employs contiguous organic and inorganic active layers, presents the problem of obtaining good adhesion between two layers of substantially dissimilar materials in a unitary element.
In addition to the above-noted problems and shortcomings associated with prior art multi-active photoconductive elements, it should be noted that, to applicant's knowledge, the art, to date, has generally disclosed no type of multi-active photoconductive element which uses and takes advantage of the above-mentioned high-speed aggregate photoconductive compositions described in Light, U.S. Pat. No. 3,615,414, except as may be described in Seus, U.S. Pat. No. 3,591,374 issued July 6, 1971. The aforementioned Seus patent describes a photoconductive element employing an aggregate photoconductive composition overcoated with a solution of a sensitizing dye of the type useful in preparing the initial aggregate photoconductive composition, ie., a pyrylium-type dye salt, whereby the overcoated dye imbibes into and interacts with the aggregate photoconductive composition to provide an increase in electrophotographic speed of the resultant aggregate composition. In this regard, it is also noted that, Mey, U.S. Ser. No. 534,978, filed Dec. 20, 1974, and cross-referenced hereinabove, described a type of multi-active photoconductive element which includes a layer employing the aggregate photoconductive compositions described in U.S. Pat. No. 3,615,414 together with an inorganic photoconductor-containing layer.
Because of the commercial need for improved aggregate photoconductive compositions, particularly those exhibiting higher electrical speeds as well as improved physical and electrical properties, e.g., lower scum formation, easier cleaning, greater resistance to wear and abrasion, improved panchromatic response, improved resistance to electrical fatigue, and improved resistance to white spot defects under ambient relative humidity conditions, it would be advantageous to develop new types of photoconductive elements which employ and improve on the existing aggregate photoconductive compositions.